German Pat. No. 23 53 884 shows a power boost device wherein a worm shaft coacts with a piston through a ball thread. The piston extends outwardly of a cylinder and means are provided to prevent relative rotation between the piston and the cylinder. In the construction where a piston rod passes through a cylinder end wall cap, the piston rod and the aperture through which it passes have a polygon profile to prevent rotation which would be undesirable in the type of mechanism shown in that patent.
The manufacture of polygon profile matching parts is relatively expensive, and also has the disadvantage that wear between the rubbing surfaces can cause a certain amount of rotational play which is undesirable. Additionally, a special designed seal for the piston rod extension is required.